1. Field of Invention
The present invention is generally related to amplifiers and more particularly to amplifiers of a push-pull type.
2. Description of the Related Art
An amplifier modulates current from a power supply responsive to an input signal. Amplifiers are divided into classes based on the circuit basics by which the modulation of the power supply is achieved.
A class “A” amplifier utilizes one or more devices each of which operate over the whole input signal cycle to deliver the amplified output signal. The Class “A” amplifier is characterized by relatively low clipping and relatively high power consumption. Relatively high power consumption is a result of the “always on” characteristic of the devices which form the Class “A” amplifier in which power consumption is essentially constant whether or not there is an input signal.
The Class “B” amplifier provides a more power efficient alternative in which complementary push and pull devices operate in anti-phase with one another on opposite halves of the input signal to deliver complementary positive going and negative going amplified portions of the output signal. During the positive going portion of the input signal the push device(s) source current to the output from a positive power supply. During the negative going portion of the input signal the pull device(s) sink current from the output to a negative power supply. The most noticeable price associated with the reduced power consumption of this Class of amplifiers is cross-over distortion or cross-over clipping. This results in the input signal region proximate the common mode or cross-over voltage at which both the push as well as the pull device(s) are in the “Off” state and thus unresponsive to the input signal.
The Class “AB” amplifier reduces the extreme cross-over distortion of the pure Class “B” architecture by biasing each into an “never off” state thereby reducing the efficiency of the amplifier. Both the push and pull devices are provided with input signal biasing to prevent their entry into the “Off” state. The Class “AB” design thus blends the characteristics of the Class “A” and Class “B” designs. This composite design reduces cross-over clipping of the pure Class “B” design at the expense of an increased power consumption associated with the pure Class “A” design. This increased power consumption arises from what is known as the ‘quiescent’ or ‘bias’ current which flows from positive to negative power supply through both the push and pull devices constantly even in the absence of an input signal. The Class “AB” architecture is attractive because only small amounts of quiescent/bias current are required to greatly reduce cross-over distortion. Thus this composite design does not for practical purposes have to take on the extreme power consumption associated with the pure Class “A” design.
Typical Class “AB” amplifiers utilize complementary transistor architectures, i.e. “npn” and “pnp” architectures, a.k.a. “N” and “P” type devices respectively. In a bipolar junction transistor (BJT) the three letters refer to the semiconductor material and hence majority charge carrier from which each region of the BJT, i.e. the emitter, base, and collector are made. Similarly in a field effect transistor the three letters refer to the semiconductor material and majority charge carrier from which the drain, substrate, and source are made. The “N” and “P” types are said to be complementary in that their switching characteristics are complementary. The “P” type device, a.k.a. “pnp” type device is generally “On” and conducting when the control input, i.e. the base of a BJT or the gate of a FET is pulled low. Conversely, the “N” type device, a.k.a. “npn” type device is generally “On” and conducting when the control input, i.e. the base of a BJT or the gate of a FET is pulled high.
The designer of a Class “AB” amplifier is faced inherent performance differences when designing a push-pull type amplifier. The most basic of these performance differences arises from the fact that holes have a much lower mobility than electrons. Thus “N” type devices, relying as they do on electrons as the current limiting charge carriers, generally have higher current carrying, gain and frequency characteristics than “P” type devices. These differences in mobility and other differences in device architecture between “P” and “N” type devices make it difficult to minimize distortion across the full operating range of the amplifier.
What is needed is an amplifier architecture which allows for reduced distortion and power consumption.